1. Field of the Invention
The present invention relates to a vacuum pump used in an apparatus such as a semiconductor manufacturing apparatus, an electron microscope, a surface analysis apparatus, a mass spectrograph, a particle accelerator, and a nuclear fusion experiment apparatus, and, more particularly, to the structure of an inexpensive vacuum pump which has a large pumping capacity and can be handled easily.
2. Description of the Related Art
In a process such as dry etching, chemical vapor deposition (CVD), or the like performed in a high-vacuum process chamber in a semiconductor manufacturing step, a vacuum pump such as a turbo-molecular pump is used of producing a degree of high vacuum in the process chamber by exhausting gas from the process chamber.
As shown in FIG. 6, in the conventional turbo-molecular pump P6, a plurality of rotor blades 17 are provided on the outer wall of a cylindrical the rotor 16, a plurality of stator blades 18, which are positioned and fixed between rotors 17, are fixed on the inner wall of the pump case 11, and the rotor 16 is integrally secured to the rotor shaft 15.
The process chamber connected to a gas suction port 12 at the top of the pump case 11 is in a high vacuum state. By driving a drive motor 19 so as to rotate the rotor shaft 15 at high speed, gas taken in from the gas suction port 12 is fed to a thread groove pump mechanism portion as the lower stage of the turbo molecular pump by the interaction between the rotor blades 17, rotating at high speed together with the rotor shaft 15, and the stator blades 18, compressed from an intermediate flow state to a viscous flow stated by the interaction between the cylindrical surface of the outer wall of the rotor 16 and thread grooves 21 on the inner wall of a threaded stator 20, and then discharged from a gas exhaust port 13 as the final stage of the turbo molecular pump P6. During this operation, since the temperature of the rotation body composed of the rotor 16 and the rotor blades 17 is increased by the heat of gas compression, it is necessary to cool the rotation body by dissipating the heat in the rotation body to stationary components in the pump case 11.
Heat radiation and heat transfer are well known as means for dissipating the heat in the rotation body. The former is performed by means (a) which radiates the heat from the rotor blades 17 to the stator blades 18, and the latter is performed by means (b) which transfers the heat by conduction via gas or means (c) which transfers the heat by conduction via bearings. However, as shown in FIG. 6, in the turbo molecular pump P6 in which the rotor shaft 15 is supported in a non-contacting manner by magnetic bearings composed of radial electromagnets 22 and axial electromagnets 23, since the rotor shaft 15 does not come into contact with protection ball bearings 24 during normal operation of the turbo molecular pump P6, the heat dissipation is not realized by the direct heat transfer via bearings of the means (c) but achieved by the heat radiation and the heat transfer via gas of the means (a) and (b), respectively. Further, when an amount of gas flowing in the pump case 11 is small or a low-thermal-conductivity gas such as Ar gas is pumped, the heat transfer via gas of the means (b) cannot be expected. Thus, only the heat radiation of the means (a) is dependable means for dissipating the heat, thereby resulting in poor heat-dissipation efficiency and accordingly causing the heat of compression of the gas pumped by the turbine to be likely stored in the rotor blades 17.
To solve this problem, as shown in FIG. 6, so far the rotating cylindrical body composed of the rotor 16 and the rotor blades 17 has been cooled by feeding a high-thermal-conductivity purging gas such as nitrogen gas (i.e., N2 gas) into the pump case 11 from the outside. More particularly, as FIG. 6 shows the flows of the vacuum-pumping gas and the purging gas indicated by the dotted and solid arrow lines, respectively, the purging gas flows along a passage R, which is in communication with the gap between the outer wall of the rotor shaft 15 and the inner wall of a stator column 14 and with the other gap between the outer wall of the stator column 14 and the inner wall of the rotor 16, and exits from the gas exhaust port 13, thereby the heat of gas compression stored in the rotor 16 being dissipated from the inner wall of the rotor 16 to the outer wall of the stator column 14.
According to this method, in order to improve the cooling effect, a gap g1 between the inner wall of the rotor 16 and the outer wall of the stator column 14 is required to be as small as possible. That is because, if the gap g1 is large, a thermal boundary layer is produced in a viscous flow region, thereby lowering the thermal conductivity of the purge gas between the inner wall of the rotor 16 and the outer wall of the stator column 14, and also if the gap g1 becomes larger than an average free path of gas molecules in a molecular flow region, the probability in which the gas molecules released from the surface of the rotor 16 directly reaches the surface of the stator column 14 becomes lower, thereby lowering the thermal conductivity of the purge gas in the same fashion as described above.
However, as shown in FIG. 7, in a turbo molecular pump P7 in which, when a rotor 16-1 having rotor blades 17-1 having a larger diameter L7 than the rotor blades 17 having a diameter L6 shown in FIG. 6 is mounted on the rotor shaft 15 shown in FIG. 6 so as to pump a larger amount of gas, the rotor 16-1 and the stator column 14 have a very large gap g2 between the inner wall of the rotor 16-1 and the outer wall of the stator column 14, compared to the small gap g1 shown in FIG. 6 between the inner wall of the rotor 16 and the outer wall of the stator column 14. Since such a large gap g2 causes the purging gas to have a dramatically lowered thermal conductivity as described above, it is required to make the gap g2 smaller down to the predetermined gap g1 by forming the outer-wall shape of the stator column 14 based on the inner-wall shape of the rotor 16-1 so as to achieve a desired thermal conductivity of the purging gas.
As a method for making the gap g2 smaller, forming the rotor 16-1 so as to have a thick lower part when manufacturing is considered. However, the thicker the lower part, the higher the cost of the rotor 16-1 becomes. In addition, since the rotor 16-1 is a high-speed rotating component during operation of the turbo molecular pump, the thicker lower part leads to the heavier rotor 16-1, and thus the turbo molecular pump requires a larger power for its operation, thereby resulting in a deteriorated compression performance and likely causing the rotation body to rotate in an unbalanced state.
As another method for making the gap g2 smaller, forming the stator column 14 so as to have an outer-wall shape based on the inner-wall shape of the rotor 16-1 is considered. However, in this case, several types of the stator columns 14, having different outer-wall shapes and accommodating expensive electrical components and the like therein, must be prepared and disposed in the pump case 11 depending on the inner-wall shape of the rotor, thereby causing a dramatic cost increase in manufacturing the turbo molecular pump.
The present invention has been made in view of the above-described problems. Accordingly, it is an object of the present invention to provide a vacuum pump in which, when a rotor having a large diameter is mounted so as to pump a large amount of gas, a small gap is easily formed, with a small amount of additional cost, between the inner wall of the rotor and the outer wall of a stator column, and which achieves a dramatic cost reduction in manufacturing the vacuum pump compared to the manufacturing cost of the conventional vacuum pump.